1. Technical Field
The present disclosure relates a method for operating a fuel cell system and a method for estimating the composition of fuel used in a fuel cell system.
2. Description of the Related Art
International Publication No. WO 2013/001753 discloses a fuel cell system including a high-temperature operating fuel cell; a plurality of fuel supply lines for supplying several types of fuels to the fuel cell, properties of the fuels, that is, the composition and calorific value per unit volume of the fuels varying, the fuels being different in properties from each other; a mixing tube which merges with the fuel supply lines and which is connected to the fuel cell; and a fuel mixture property measurement unit which is attached to the mixing tube and which measures the flow rate and properties of a fuel mixture. A steam pipe including a steam flow control valve is connected to the upstream side of a reformer which includes a current density control unit that controls the current density and which is attached to the mixing tube. A fuel cell control unit which receives a signal from the fuel mixture property measurement unit to control the steam flow control valve is placed. The composition, calorific value per unit volume, and flow rate of the fuel mixture are measured by the fuel mixture property measurement unit. The fuel cell control unit determines the content of carbon in the fuel mixture from the ratio between higher hydrocarbon components contained in the fuel mixture, determines the content of hydrogen in the fuel mixture from the supply of steam at that point in time to compute the ratio of carbon to hydrogen, and judges whether the computed ratio of carbon to hydrogen is within the range of 1:5 to 1:15. When the computed ratio of carbon to hydrogen is outside this range, the fuel cell control unit controls the steam flow control valve to regulate the supply of steam such that the ratio of carbon to hydrogen in the fuel mixture is within this range. When the computed ratio of carbon to hydrogen is within this range, the fuel cell control unit maintains the supply of steam.
International Publication No. WO 2013/153944 discloses a method for controlling the mass flow rate of steam in a hydrogen production plant that includes a mass flowmeter measuring the mass flow rate of a source gas and also includes a control processor which computes the mass flow rate of carbon contained in the source gas on the basis of the mass flow rate measured by the mass flowmeter and which outputs a signal for controlling the flow rate of steam to a steam flow regulator on the basis of the mass flow rate of carbon. The method includes allowing the control processor to calculate the mass flow rate of carbon contained in the source gas on the basis of the ratio of the mass flow rate measured by the mass flowmeter to the approximate mass of carbon supposed to be contained in the source gas, allowing the control processor to correct the computed value with a correction value preset to the source gas, and allowing the control processor to control the mass flow rate of steam on the basis of the corrected value.